STP brokers sit between the client and the interbank market. They send your order directly to one of several liquidity providers, pocket a small markup on the spread, and pass the rest through. There is no dealing desk intervening on individual orders.
Compared to ECN, STP is simpler for beginners — one price, no separate commission. Compared to a dealing desk (market-maker) model, STP reduces conflict of interest because the broker profits from your volume, not your losses.